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Physicist, University of Vienna; Scientific Director, Institute of Quantum Optics and Quantum Information; President, Austrian Academy of Sciences; Author, Dance of the Photons: From Einstein to Quantum Teleportation

There is No Reality in the Quantum World

The idea to be abandoned is the idea that there is no reality in the quantum world. The idea probably came about because of two reasons. On the one hand, because of the fact that one cannot always ascribe a precise value to a physical property, and on the other hand, because within the wide spectrum of interpretations of quantum mechanics some suggest that the quantum state does not describe an external reality, but rather that the properties only come about in the mind of the observer and therefore that consciousness plays a crucial role.

Let us consider for a second the famous double-slit experiment. Such experiments or their equivalents have to date not only been performed with single photons or any other kind of single particles, like neutrons, protons, electrons etc., but even with very large macromolecules, such as buckyballs and even larger. Specifically we do the experiment with buckyballs—the C-60 or C-70 molecules. You have two slits and under the right experimental conditions, you observe a distribution of the buckyballs behind the slits which has maxima and minima, the interference pattern. This is due to interference of the probability waves passing through both slits. But, following Einstein in his famous debate with Niels Bohr, we might ask if we do the experiment with individual particles, individual buckballs one by one: Through which slit does an individual buckyball molecule pass? Would it not be natural to assume that every particle has to pass either slit? Quantum physics tells us that this is not a meaningful question. We cannot assign a well-defined position to the particle unless we actually perform an experiment which allows us to find out where it is. So, before we do the measurement, the position of the buckyball—and therefore the slit it passes through—is a concept devoid of any meaning.

Suppose we now measure the position of the particle. Then we get an answer and know where it is. It is either near one slit or near the other slit. In that case, position is certainly an element of reality, and we can clearly say that quantum physics describes this reality. What is interesting is that having precise knowledge of one feature, namely the position, another kind of knowledge, namely the one encoded in the interference pattern, is not well-defined anymore.

Where could consciousness come in here? Quantum mechanics tells us that the particle, before any observation, is in a superposition of passing through one slit and of passing through the other slit. If we now have two detectors, one each behind each slit, then either detector will register the particle. But quantum mechanics tells us that the measurement apparatus becomes entangled with the position observable of the particle, and thus itself does not have well-defined classical features, at least in principle. This, following the Hungarian-American Nobel prize winner Eugene Wigner, is a chain which can be followed until an observer registers the result. So if we would adopt that reasoning, it is the consciousness which would make reality happen.

But you don't have to go so far. It is enough to assume that quantum mechanics just describes probabilities of possible measurement results. Then making an observation turns potentiality into actuality and, in our case, the position of the particle becomes a quantity one can talk reasonably about. But, whether it has a well-defined position or not, the buckyball very well exists. It is real in the double-slit experiment, even when it is impossible to assign its position a well-defined value.